A. Field of Invention
This invention relates to idlers for continuously variable transmissions; specifically to such idlers that have more than one axis of rotation.
B. Description of the Prior Art
Most transmissions, such as those found on bicycles with derailleur gear-shifting mechanisms, or those found in manual or automatic shift motor vehicles, suffer three distinct disadvantages: 1) they offer only a limited number of fixed gear ratios; 2) these ratios can be changed only by disengaging the transmissions' driving and driven members, thereby temporarily halting the transfer of torque; and 3) such transmissions lose a significant amount of power to friction.
Continuously variable transmissions solve the first of these problems by offering a potentially infinite range of nondiscrete input/output torque ratios (or "gear" ratios). But most continuously variable transmissions still must either have driving and driven members disengaged during shifting or suffer from high frictive losses and wear. Although the toric type of continuously variable transmission can shift with the driving and driven members engaged, it requires a relatively complex shifting mechanism that cannot function when the transmission rests. See, for example, "Subaru Revives the Variable Transmission," Consumer Reports (August, 1989) pp. 500-501; Lubomyr O. Hewko, "Automotive Traction Drive CVT's--An Overview," SAE Technical Paper Series, No. 861355 (1986); Stuart H. Loewenthal and Erwin V. Zaretsky, "Design of Traction Drives," NASA Reference Publication, No. 1154 (1985); ME Staff Report, "`Rubber-Band` Cars are Coming Back," Mechanical Engineering, (Dec. 1984) pp. 34-38; S. H. Loewenthal, D. A. Rohn, and N. E. Anderson, "Advances in Traction Drive Technology," SAE Technical Paper Series, No. 831304 (1983).
The problem with these continuously variable transmissions lies in the idlers that they use. The idler of a continuously variable transmission conveys torque from the driving to the driven member of the transmission. It also changes the input/output torque ratio of the transmission by moving laterally across the rotating surfaces of the transmission's driving and driven members. The idler's lateral movement thus "shifts the gears" of a continuously variable transmission.
The idlers of most continuously variable transmissions are "uniaxial"; that is, they can rotate around only one axis at a time. This causes problems. Most continuously variable transmissions with uniaxial idlers can shift gears only by disengaging the idler from the driving and driven members (thereby halting the transfer of torque) and then moving the idler laterally. In the alternative, these transmissions shift gears by simply dragging the idler laterally across the surfaces of the driving and driven members (thereby causing significant frictive losses and undue wear). The toric continuously variable transmission, when equipped with a uniaxial idler, shifts only with the help of relatively complex machinery that inclines the idler and steers it across the transmission's driving and driven members as they rotate. This process fails to work when the driving and driven members rest.
An idler could avoid these problems by rotating around two or more substantially orthogonal axes at a time. Such an idler could roll laterally across the surfaces of the driving and driven members whether or not they are rotating. There would thus be no need to halt the transfer of torque while shifting gears, little power would be lost to friction, and wear would decrease. When used in a toric continuously variable transmission, such an idler could shift through a relatively simple process that works both when the transmission runs and when it does not.
Some inventors have tried to capture these advantages by designing idlers with multiple axes of rotation. See U.S. Pat. No. 3,270,576, filed February 1964 by D. Goldwater; and U.S. Pat. No. 4,448,087, filed March 1981 by E. Barzel. Such multiaxial idlers can, indeed, rotate simultaneously around several substantially orthogonal axes. But these designs incorporate many more axes than absolutely necessary. Having so many extra axes renders these multiaxial idlers unnecessarily complex, hard to build, prone to break, difficult to repair, and heavy.
Furthermore, the need to accommodate multiple axes tends to leave gaps along the edge of a multiaxial idler, exactly where the idler comes into contact with the driving and driven members. These gaps can cause a multiaxial idler to transmit torque roughly and discontinuously. U.S. Pat. No. 4,448,087 offers a design without such gaps, but only at the cost of additional complexity.
These limitations in the prior art of idler design betray a commonplace--but nonetheless erroneous--assumption. Examples from physics textbooks and familiar machinery encourage inventors to assume that solid bodies rotate only around rigid, straight axes. As described more fully below, however, this need not be the case.